Asymmetric photodestruction of bicyclo[2.2.2]oct-2-ene-5,7-dione: absolute configuration and 1n-π* excited state geometry

Abstract

Racemic bicyclo[2.2.2]oct-2-ene-5,7-dione (1) is partially resolved by asymmetric photodestruction with circularly polarized light. The enantiomeric purity of (1) and of its rearranged photoproduct bicyclo[4.2.0]oct-3-ene-2,7-dione (4) can be determined using a convenient formalism, essentially due to Kuhn. The absorption and c.d. of the βγ-unsaturated 1,4-dione (1) are reported and discussed from theory. They can be quantitatively explained in terms of an interaction of the coupled local carbonyl n→π* transitions with the ethylenic π→π* transition. This not only permits the assignment of the absolute configuration of (1)and, by chemical correlation, that of (4), it also opens away to interpret the fluorescence data of (1). The emission probably originates from a locally excited carbonyl group. The circular polarization in the fluorescence indicates that the carbonyl group is no longer planar but pyramidal. The CO out of-plane angle is ≲40° in the direction of the ethylenic bond. The c.d. of (4), which is conformationally flexible and contains both an αβ-unsaturated and a saturated carbonyl chromophore, allows for a tentative determination of the preferred conformation.